Transfer sheets having a tab (tabbed transfer sheets) have been used for various image forming apparatuses, such as copiers and printers.
FIG. 4 illustrates tabbed transfer sheets arranged between non-tabbed transfer sheets having no tab In FIG. 4, seven transfer sheets are aligned, wherein the third transfer sheet from the front (X in FIG. 4), and the fifth transfer sheet (Y in FIG. 4) are “tabbed transfer sheets”.
Further, in FIG. 4, the protrusion x of the third transfer sheet from the front (X in FIG. 4) is a tab, and the protrusion y of the fifth transfer sheet from the front (Y in FIG. 4) is a tab. In FIG. 5, L represents the tab length and W represents the tab width. In FIG. 4, the tabs x and y are arranged at different positions, with respect to the longitudinal side of the transfer sheets, which are the tab positions.
Further, FIGS. 6a to 6d show examples of types of tabs. The tabs shown in FIG. 6a have the same tab width, and different tab lengths (pattern A). The tabs are formed in shapes which divide the side, of the transfer sheets, provided with the tabs into three parts (3 tabs). That is, the number of division in this case is three.
Tabs (pattern A), shown in FIG. 6b, having the same tab width and different tab lengths are formed in shapes (four tabs) which divide the side of the transfer sheets provided with the tabs into four parts. That is, the number of division is four in this case.
Tabs (pattern B), shown in FIG. 6c, having the same tab width and length and different positions (tab positions) where the tabs are provided, are formed in shapes (three tabs) which divide the side of the transfer sheets provided with the tabs into three parts. That is, the number of division is three in this case.
Further, tabs (pattern B), shown in FIG. 6d, having the same tab width and tab length and different positions (tab positions) provided with the tabs, are formed in shapes (four tabs) which divide the side provided with the tabs into four parts. That is, the number of division is four in this case.
Incidentally, even when the pattern of tabbed transfer sheets is the same, and the above number of division is also the same for example, since the tab positions, tab lengths, or tab widths are different, a conventional image forming apparatus is incapable of detecting tab positions.
Further, in order to prevent image forming from extending out into a tab area, it was necessary to set tab sheets on a sheet feeding cassette in a state that the tab positions are correct. In fact, regarding image forming on a tab area, it was impossible to confirm the state without actually forming an image.
Further, since tabbed transfer sheets are intended to be inserted as inter-sheets, it is necessary to set tabbed transfer sheets on a sheet feeding cassette in an insertion order such that the tab positions are correct. In other words, when tabbed transfer sheets are set with a wrong insertion order with respect to tab positions, tabbed transfer sheets with desired tab positions are not inserted, which does not allow even detecting the state.
For image forming apparatuses that handle these types of tabbed transfer sheets, offers have been made regarding tab detection and control of sheet feeding timing in the following Patent Documents 1 and 2, and others.    Patent Document 1: Japanese Patent Application Publication TOKKAI No. H11-225247 (page 1, FIG. 1)    Patent Document 2: Japanese Patent Application Publication TOKKAI No. H8-245003 (page 1, FIG. 1)
According to inventions disclosed in the above Patent Documents, it is possible to perform tab detection and tab control to some extent. However, a tab detection sensor having multiple devices is required to accurately detect tab positions and tab types. Therefore, a complicated mechanism is required sp as to arrange a tab detection sensor having multiple devices, and it is also necessary to provide a detection circuit with multiple input ports to receive detection results by the tab detection sensor having multiple devices.
In recent years, with various image forming apparatuses, such as copiers and printers, it has been attempted to perform image forming by the use of “unprocessed tabbed transfer sheet blanks” which can be processed into tabbed transfer sheets having a tab (tabbed transfer sheets) by post-processing. Hereinafter, an unprocessed tabbed transfer sheet blank means a transfer sheet that can be processed into a tabbed transfer sheet having a tab by post-processing.
FIGS. 7a and 7b illustrate the state of unprocessed tabbed transfer sheet blanks. Herein, FIG. 7a shows the state of an unprocessed tabbed transfer sheet blank before post-processing. Herein, the transfer sheet is in a rectangular shape in a larger size than a regular size such as to contain the area to be processed into a tab and the peripheral area.
FIG. 7b shows the state that an unprocessed tabbed transfer sheet blank has become a tabbed transfer sheet after post-processing. Herein, the area that was prepared for processing into a tab remains as a tab, while the unnecessary peripheral area has been trimmed by a post-processing device. As a result, now in the state after post-processing, the transfer sheet is the same as a regular tabbed transfer sheet in the shape and size.
Such unprocessed tabbed transfer sheet blanks are not formed with an actual tab at the time of sheet feeding and image forming on an image forming apparatus. Accordingly, there is a problem that conventional tab detection is not practical at all in this regard.
The present invention has been devised to overcome problems, such as described above, and an object of the invention is to realize an image forming apparatus capable of reliably detecting tab positions and tab types and image forming in a state that tab sheets are inserted correctly, with a simple control, regardless of tabbed transfer sheets or unprocessed tabbed transfer sheet blanks.